Re-define the meaning of "Killing Blow" - Support Gem does not "show" function of stolen Modifiers

Header says all. But the Problem seems to refer to the Combination of Gems like "Volcanic Eruption" let you deal "Projectile Damage" by "Striking" - what probably is not affected by the "Killing Blow".

Here some emotional Information:
I dont know what several Modifier-Buffs from a LAST HIT with my MACE STRIKE, do as I would need it, due to the lack of readable information on the 20-second buff. If a "Killing Blow" means something different, let us know the redefinition by underlining the "Function" because i did something around 25 Kills, but did not recognize anything.

Update: It also seems like, if i get one of these Modifiers, there is a Chance to receieve one of my own "Debuffs" back at stealing. What results in some Cases as a One-Shot to myself by receiving a "Heavy Stun", "Exposure" "Resistance Reduction" or something else what i cant need at all...

Greetings.